This invention relates to a process for synthesizing thermoplastic block copolymers by selective cyclization. The process involves chemical modification of polymers containing at least two different blocks, at least one of which is derived from a suitable conjugated diene monomer, as defined hereafter. The polymers may contain other blocks derived from monomers such as styrene, t-butylstyrene, vinyl toluene, alpha-methylstyrene, 4-vinylbiphenyl, methacrylonitrile, and caprolactone.
Block copolymers such as polystyrene-polybutadiene-polystyrene (SBS) copolymers are well-known in the art (see U.S. Pat. No. 3,149,182, Canadian Pat. No. 746,555, and British Pat. No. 895,980).
It is also known that thermoplastic block copolymers such as the SBS copolymers behave as self-reinforced, vulcanized elastomers. The term "self-reinforced" means that the block copolymer has the stress-strain and other physical characteristics (apart from sensitivity to certain solvents) of vulcanized rubbers.
Block copolymers composed substantially of conjugagated diene monomers, for instance, isoprene and butadiene, have properties resembling those of unvulcanized rubber. They are soft, tacky materials having very low tensile strength.
Self-reinforcing block copolymers prepared from diene block copolymers having the structure C-D-C wherein each C is a homopolymer of a branched chain conjugated diene and D is a homopolymer block of an unbranched (straight chain) conjugated diene are described in U.S. Pat. No. 3,452,118. That patent discloses a thermoplastic elastomer synthesized by selective hydrochlorination of a polyisoprene-polybutadiene-polyisoprene (IBI) block copolymer. Selective hydrochlorination of these diene block copolymers produced rubbery, high strength ("self-reinforcing") block copolymers. During this reaction, the polyisoprene block became hydrochlorinated while the central rubbery block was substantially unchanged (see also British Pat. No. 1,163,674).
Hydrogenation has also been used to modify block copolymers: improve stability (U.S. Pat. No. 3,700,748); improve physical properties (U.S. Pat. Nos. 3,333,024 and 3,823,203); increase oxidation resistance and improve creep and set characteristics (U.S. Pat. No. 3,595,942); and improve processibility (U.S. Pat. No. 3,700,633). U.S. Pat. No. 3,985,826 and British Pat. No. 1,160,234 describe the synthesis of thermoplastic block copolymers by hydrogenation of a BIB-type block copolymer. Hydrogenated SBC block copolymers (where C is a random styrene-butadiene type copolymer block) are disclosed in U.S. Pat. No. 3,792,127.
Processes for preparing block copolymers from conjugated diene monomers and controlling the microstructure of the resultant polymers are disclosed in U.S. Pat. Nos. 3,830,880 and 3,140,278.
A self-reinforcing thermoplastic block copolymer which has relatively high tensile strength and solubility in a wide variety of solvents has many application possibilities. Desired properties such as elasticity and low permanent set or residual strain, tear strength, and resistance to moisture permeation can be built into block copolymers by adjusting such variables as overall molecular weight, molecular weight of the individual blocks, weight ratio of the blocks, and the types of blocks used. Thus, block copolymers can be used in such varied applications as coatings, films, adhesives, rubber bands, shoe soles, rubber hose, automotive parts, impact-resistant plastics, and artificial leather.
Although hydrogenation and hydrochlorination (as mentioned previously) have been utilized in modifying diene block copolymers, the fact that polydienes are known to cyclize has not heretofore been utilized to achieve self-reinforcing block copolymers. A possible explanation is that previous studies have indicated significant disadvantages. For instance, polyisoprene is reported to undergo chain scission when it is exposed to cyclization conditions. In the case of polybutadienes, substantial crosslinking and gel formation accompanies cyclization. Thus, it was quite unexpected that IBI-type block copolymers could be cyclized to essentially soluble products with very little evidence of chain sission or other disadvantages.
Cyclization of homopolymeric elastomers is old in the art. The production of a resin from natural rubber by cyclizing it with stannic chloride or chlorostannic acid has long been known. Cyclization reactions of natural and synthetic rubbers are reported in J. J. Janssen, Rubber Age, 78, 718 (1956); M. Stolka, J. Vodehnal, and I. Kossler, J. Polym. Sci.A, 2, 3987 (1964); and M. A. Golub and J. Heller, Can. J. Chem., 41, 937 (1963).
The cyclization of diene polymers has been discussed by M. A. Golub, "Cyclized and Isomerized Rubber," Chapter 10 in Polymer Chemistry of Synthetic Elastomers, Kennedy and Tornqvist, eds., John Wiley & Sons, 1969.
The experimental work on which this specification is based has shown that only specific cationic catalysts and catalyst/cocatalyst combinations are capable of achieving selective cyclization-free of undesirable side reactions of block copolymers derived from conjugated diene monomers.